Synthesis of sterically-stabilized polystyrene latexes using well-defined thermoresponsive poly(N-isopropylacrylamide) macromonomers

A series of well-defined thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) macromonomers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization followed by aminolysis and nucleophilic Michael addition. 2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic-2-phenoxyethyl ester (CTA) was used as the RAFT chain transfer agent to prepare six PNIPAM-CTA precursors with target degrees of polymerization of 20, 30, 40, 50, 60, and 75. These NIPAM polymerizations were conducted in 1,4-dioxane and proceeded with good control and low polydispersities (M_w/M_n < 1.10) up to more than 90% conversion. The PNIPAM trithiocarbonate end-groups were then converted to methacrylate end-groups by combining (i) aminolysis and (ii) nucleophilic Michael addition using the bifunctional reagent, 3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHPMA), in a one-pot reaction. The resulting PNIPAM macromonomers were evaluated as reactive steric stabilizers for latex syntheses. Near-monodisperse submicrometer-sized latexes were obtained by alcoholic dispersion polymerization of styrene in methanol, as judged by scanning electron microscopy (SEM) and dynamic light scattering (DLS). In contrast, a latex synthesized in the presence of a PNIPAM-CTA had a bimodal size distribution, while thiol-capped PNIPAM chains produced ill-defined nonspherical particles and styrene polymerization conducted in the absence of any stabilizer led to macroscopic precipitation. These control experiments confirm that using the methacrylate-capped macromonomers is essential for successful latex syntheses. ¹H NMR analysis confirm the presence of PNIPAM chains in the latex particles and XPS measurements indicate that the stabilizer is located on the particle surface, as expected. The well-known thermo-responsive nature of the stabilizer was successfully transferred to these latexes, which exhibit reversible flocculation upon heating above the LCST of the PNIPAM chains.